Such changes in wavelength are used in particular to solve problems of conflict when routing signals.
In such converters, the information is often in the form of binary data represented by pulses modulating an optical carrier wave. A binary value is thus determined as a function of the amplitude (or power) level of the modulated lightwave.
During transmission, the signal can be subjected to degradation that makes it more difficult for receivers to detect the high levels and the low levels of the received signal.
In the amplitude domain, the quality of an optical signal is usually defined by at least two parameters: the signal-to-noise ratio and the extinction ratio.
The signal-to-noise ratio is defined as the ratio of the optical power of the signal to the noise power in a wavelength band including the wavelength of the carrier of the signal.
The extinction ratio is defined as the power ratio corresponding to the high levels of the signal divided by the low levels of the signal. This ratio must be high enough in spite of variations in the input signal.
One common way of making a wavelength converter consists in using an interferometer structure of the Mach-Zehnder type or of an equivalent type.
Such an interferometer structure is shown in FIG. 1. It is made up of two guiding branches 1 and 2. At least one of the branches is provided with a semiconductor optical amplifier OA.sub.1. In general, for reasons of symmetry, it is preferable to place a second semiconductor optical amplifier OA.sub.2 on the other branch 2 as well. The presence of the second semiconductor optical amplifier OA.sub.2 makes it possible to retain substantially the same amplification level in both of the branches of the structure, and therefore to have substantially identical power levels at the outputs of the branches of the interferometer.
A first coupler K.sub.1 makes it possible to couple one end of each of the branches to a peripheral semiconductor optical amplifier, also referred to as an "input" amplifier, OA.sub.5. A laser source 7 makes it possible to deliver an output carrier wave M of wavelength .lambda..sub.s to the peripheral amplifier OA.sub.5.
A second coupler K.sub.2 is disposed so as to couple the other end of the first branch 1 to another input peripheral semiconductor optical amplifier OA.sub.4. The coupler K.sub.2 enables an input signal E to be fed into the first amplifier OA.sub.1, the input signal being of wavelength .lambda..sub.e and having been amplified by the input amplifier OA.sub.4.
A third coupler K.sub.3 connected to the coupler K.sub.2, to the second amplifier OA.sub.2, and to another peripheral semiconductor optical amplifier OA.sub.3 for output purposes, is disposed so as to deliver an output signal S resulting from coupling of the auxiliary waves AM.sub.1, and AM.sub.2 delivered by respective ones of the first and second amplifiers OA.sub.1 and OA.sub.2. The waves AM.sub.1, and AM.sub.2 correspond to the waves M.sub.1 and M.sub.2 output by the coupler K.sub.1 and amplified by respective ones of the amplifiers OA.sub.1 and OA.sub.2. The output signal S, of wavelength .lambda..sub.s, is then amplified by the output peripheral amplifier OA.sub.3.
Another peripheral amplifier OA.sub.6 is also provided to keep the structure symmetrical and to enable either one of the amplifiers OA.sub.3 or OA.sub.4 to be replaced in the event of amplifier failure.
Currents I.sub.1 and I.sub.2 are injected into respective ones of the amplifiers OA.sub.1 and OA.sub.2 via electrodes E.sub.1 and E.sub.2. The output signal S results from the waves AM.sub.1, and AM.sub.2 interfering constructively or destructively, depending on the phase difference between the two branches of the interferometer.
To obtain effective wavelength conversion, the saturation power threshold of such an interferometer structure is set relatively low, which is favorable to conversion in the branches of the Mach-Zehnder structure but is unfavorable to the extinction ratio in the output amplifier. When the power of the signal that arrives at K.sub.3 (FIG. 1), for example, exceeds the saturation threshold, which is low, high level gain becomes smaller than low level gain, so that the high levels of the output signal S are compressed, and the signal is therefore distorted. Such distortion may also occur on the input signal E, or on the output carrier wave M. Unfortunately, if the input signal E is distorted, or if the output carrier wave M is distorted, then the output signal S is also degraded, and the extinction ratio of the interferometer structure is reduced.